The present invention relates to a liquid crystal display device, and more particularly relates to a liquid crystal display device having a memory display function for displaying an image during stopping the application of voltage to a liquid crystal material.
Along with the recent development of so-called information-oriented society, electronic apparatuses, such as personal computers and PDA (Personal Digital Assistants), have been widely used. With the spread of such electronic apparatuses, portable apparatuses that can be used in offices as well as outdoors have been used, and there are demands for small-size and light-weight of these apparatuses. Liquid crystal display devices are widely used as one of the means to satisfy such demands. Liquid crystal display devices not only achieve small size and light weight, but also include an indispensable technique in an attempt to achieve low power consumption in portable electronic apparatuses that are driven by batteries.
The liquid crystal display devices are mainly classified into the reflection type and the transmission type. In the reflection type liquid crystal display devices, light rays incident from the front face of a liquid crystal panel are reflected by the rear face of the liquid crystal panel, and an image is visualized by the reflected light; whereas in the transmission type liquid crystal display devices, the image is visualized by the transmitted light from a light source (back-light) placed on the rear face of the liquid crystal panel. The reflection type liquid crystal display devices have poor visibility because the reflected light amount varies depending on environmental conditions, and therefore transmission type color liquid crystal display devices using color filters are generally used as display devices of, particularly, personal computers that display full-color images.
As the color liquid crystal display devices, currently, active matrix liquid crystal display devices using switching elements such as TFT (Thin Film Transistor) are widely used. Although the TFT-driven liquid crystal display devices have relatively high display quality, they require a back-light with high intensity to achieve high display brightness because the light transmittance of the liquid crystal panel is only several % at present. Consequently, a lot of power is consumed by the back-light. In addition, since the responsiveness of liquid crystal to an electric field is low, there is a problem of low response speed, particularly low half-tone response speed. Moreover, since the color display is realized using color filters, a single pixel needs to be composed of three sub-pixels, and therefore it is difficult to achieve a high-resolution display and sufficient color purity in the display.
In order to solve such problems, the present inventor et al. developed field-sequential liquid crystal display devices (see, for example, T. Yoshihara, et al., ILCC 98, P1-074, 1998; T. Yoshihara, et al., AM-LCD '99 Digest of Technical Papers, p. 185, 1999; T. Yoshihara, et al., SID '00 Digest of Technical Papers, p. 1176, 2000). Since such a field-sequential liquid crystal display device does not require sub-pixels, it is possible to easily realize a higher resolution display compared to a color-filter type liquid crystal display device. Moreover, since the field-sequential liquid crystal display device can use the color of light emitted by the light source as it is for display without using a color filter, the displayed color has excellent purity. Further, since the light utilization efficiency is high, the field-sequential liquid crystal display device has the advantage of low power consumption. However, in order to realize a field-sequential liquid crystal display device, high-speed responsiveness (2 ms or less) of liquid crystal is essential.
In order to provide a field-sequential liquid crystal display device with significant advantages as mentioned above or increase the speed of response of a color-filter type liquid crystal display device, the present inventor et al. are conducting research and development on the driving of liquid crystal such as a ferroelectric liquid crystal having spontaneous polarization, which may achieve 100 to 1000 times faster response compared to a prior art, by a switching element such as TFT (see, for example, Japanese Patent Application Laid-Open No. 11/119189 (1999)). In the ferroelectric liquid crystal, the long-axis direction of the liquid crystal molecule is tilted by the application of voltage. A liquid crystal panel holding the ferroelectric liquid crystal therein is sandwiched by two polarization plates whose polarization axes are crossed Nicols to each other, and the intensity of transmitted light is changed using the birefringence caused by the change in the long-axis direction of the liquid crystal molecule. For such a liquid crystal display device, a ferroelectric liquid crystal having a half-V shaped electro-optic response characteristic to the applied voltage as shown in FIG. 1 (characteristic exhibiting high light transmittance when a voltage of one polarity is applied and exhibiting lower light transmittance (low light transmittance practically recognized as a black image) when a voltage of the other polarity is applied as compared to the application of voltage of the one polarity) is generally used as a liquid crystal material.
As described above, the field-sequential liquid crystal display device has high light utilization efficiency and can reduce power consumption compared to the color-filter type liquid crystal display device. However, a further reduction in power consumption is required for portable apparatuses that are driven by batteries. Similarly, color-filter type liquid crystal display devices are required to reduce power consumption.